TEHRAN, Iran - A captive Royal Marine was shown in new TV footage Friday apologizing for being in Iranian waters, and Tehran made public a third letter supposedly written by the only woman prisoner among 15 Britons seized by Iran's Revolutionary Guards.

Britain sharply denounced Iran over the treatment of the captives - a clear sign both sides were hardening their stance as the crisis entered its second week.

Iran appeared intent on sending a message of strength as it faces mounting U.N. Nations sanctions over its uranium enrichment program, which the U.S. and other nations suspect the Islamic Republic is using to develop nuclear weapons.

Underlining Iran's hardline sentiment, some 60,000 soccer fans chanted "Death to Britain" at a match in Tehran, while 700 people rallying near Tehran University yelled "We condemn the British invasion!" A Muslim cleric told worshippers during Friday prayers that "Britain is an aggressor and Iran has confronted it."

In the latest video broadcast by Iranian state television, Royal Marine rifleman Nathan Thomas Summers was pictured while sitting with another male captive, both in fatigues, and female British sailor Faye Turney in a blue jumpsuit and a black head scarf.

The three were among 15 British sailors and marines detained by Iranian naval units March 23 while patrolling for smugglers near the mouth of the Shatt al-Arab, a waterway that has long been a disputed dividing line between Iraq and Iran.

The video, the second broadcast of the detainees in three days, drew indignation from British Prime Minister Tony Blair, who accused Tehran of manipulating the prisoners.

"All it does is enhance people's sense of disgust. Captured personnel being paraded and manipulated in this way doesn't fool anyone," Blair said. "What the Iranians have to realize is that if they continue in this way, they will face increasing isolation."

It was not known whether the marine spoke under pressure from his captors, but Summers said in the broadcast that "our treatment has been very friendly." Iran also released a third letter supposedly written by the sole female detainee in which she says she has been "sacrificed" by Britain. She said she was sorry for straying into Iranian waters and asked if it wasn't time for Britain to withdraw its troops from Iraq. "I am writing to you as a British serviceperson who has been sent to Iraq, sacrificed due to the intervening policies of the Bush and Blair governments," the letter said, using even more stilted language than the previous letters. The European Union warned Iran of undefined "appropriate measures" if the British naval team remained in captivity and called on Iran to inform the British government of their whereabouts and give British diplomats access - a British request that Iran so far has denied. Iran's Foreign Ministry spokesman, Mohammad Ali Hosseini, condemned the warning, saying Europeans should not take a collective stance in support of Britain. He said statements by some EU officials were "unreasonable, illogical" and Iran considered them "interference in its internal affairs." The office of the Turkish prime minister, who is trying to mediate the dispute, said Iranian President Mahmoud Ahmadinejad had indicated his government was willing to reconsider freeing Turney, who is married and has a 3-year-old daughter. Iran's foreign minister, Manouchehr Mottaki, had suggested Wednesday that Turney would be freed soon, but the following day the semiofficial Iranian news agency Mehr said her release was canceled. It quoted Iran's military chief as blaming "wrong behavior" by the British government. British Foreign Secretary Margaret Beckett, who denounced Friday's video as "appalling," said a letter from Iran on the detention of the 15 sailors and marines would not help resolve the standoff. "There is nothing in the letter to suggest that the Iranians are looking for a way out," she told British Broadcasting Corp. The letter stopped short of asking for a formal apology but instead asked for Britain to acknowledge its naval team trespassed into Iranian waters and confirm it would not happen again. The sailors and marines, part of a U.N.-mandated force patrolling the Persian Gulf, were seized while searching merchant ships for evidence of smuggling. Britain insists the team was in Iraqi waters and has said no admission of error would be made. Iraqi officials have backed Britain's account, saying again Friday that the Royal Navy personnel were captured in Iraq's territorial waters. Iraqi Foreign Minister Hoshyar Zebari said his government was in contact with Iran to "ensure the wise handling of the case." Britain has frozen most contacts with Iran and taken the issue to the U.N. Security Council, which expressed "grave concern" Thursday over Iran's seizure of the Britons. Russia, which blocked a stronger council statement, called Friday for an independent U.N. inquiry. Iran said Friday that the Security Council had no place in what it called a purely bilateral dispute. "The British government's attempt to engage third parties, including the Security Council, with this case is not helpful," it said. The statement, released by the Iranian mission at the United Nations and in London, said the British and Iranian governments "have been closely examining and discussing the case in order to settle it in a mutually acceptable manner." The recent rise in world oil prices to six-month highs cooled some Friday, but traders remained jittery that the situation could lead to a disruption of petroleum shipments out of the Gulf. AP-CS-03-30-07 1643EDT